Digitizer tablets are well known in the art. In certain applications, it is necessary or highly desirable to back-light the object that is being digitized. The object to be digitized is most often a thin, flat material such as maps, drawings, photos, or other graphic data, made of paper, film, or other translucent materials. These materials are often large format (up to 44.times.60 inches in size).
Back-lighting of the material being digitized may be accomplished by using what is commonly known as a back-lit digitizer. Back-lit digitizers, like those described in U.S. Pat. No. 4,943,689 (Siefer and Purcell), have a number of undesirable characteristics when compared to the same type of digitizer in an opaque configuration (i.e. without the modifications to make it back-lit).
This back-lit digitizer, has a number of similarities with other back-lit digitizers. The grid for detecting digitizer position is built into a tablet made of translucent materials. The translucent tablet is placed on top of a box containing fluorescent lamps. This translucent tablet/light box configuration has many inherent drawbacks. These back-lit digitizers have a grid like structure of opaque electrical conductors within the working surface of the tablet. They use light diffusing materials to hide the shadows caused by the opaque conductors. They may also have a perforated metallic shield. The result is that the illuminatable area is relatively opaque and therefore inefficient at passing light. This inefficiency requires a great number of lamps, and associated lamp ballasts, relative to the light available at the working surface.
U.S. Pat. No. 4,943,689 shows a back-lit digitizer with 20 lamps, 6 ballasts and 4 cooling fans. The cooling fans create some noise which is potentially bothersome when trying to do precise digitizing for long periods of time. The fans may wear out and require periodic replacement. The heat must be absorbed by the room air conditioning system. The lamps are operated underneath the digitizing work surface so, heat from the lamps may cause expansion of the work surface which degrades accuracy.
Current back-lit digitizers also present a weight problem. A typical opaque digitizer tablet, with a 36.times.48 inches working surface may typically weigh 50 pounds and have a thickness of 1.5 inches. In contrast, a typical back-lit digitizer tablet with a 36.times.48 inches working surface may typically weigh 150 pounds and have a thickness of 8 inches.
The added thickness and weight of these back-lit digitizers present a problem of mounting the digitizer. As these digitizers are often of large format (e.g. 24.times.36 inches, 36.times.48 inches, or 44.times.60 inches) it is necessary to mount them on a pedestal which allows the user to adjust the tilt and height. Tilt and height adjustment allows the user to reach all areas of the work surface and to vary his/her position to minimize fatigue.
Standard opaque digitizing tablets are safely mounted on ordinary adjustable drafting table pedestals. To carry the additional weight of a back-lit tablet, a more sturdy and more expensive pedestal is used, which may have an electrically operated motor to raise and lower the heavy tablet.
The thickness of a back-lit tablet moves the center of mass several inches from the pivot point of the tilt mechanism on a pedestal. To safely tilt the back-lit tablet, manufacturers often recommend a pedestal with a costly motorized tilt feature.
Back-lit tablets are known to be structurally weak when compared to opaque tablets. This is because the tablet is made without a solid backing so that back-lighting will illuminate it. If a back-lit tablet is made with plastics, it may flex or droop from gravity or pressure during use. Flex or droop may degrade the accuracy of the digitizer. If a back-lit tablet is made of glass it may be less prone to flexing, but these glass tablets are fragile and have been known to break during shipment or use. The most popular method of constructing a grid array, for use in an opaque digitizing tablet, is by photo-etching a copperclad FR4 substrate (like the "printed circuit boards" used in personal computers). This type of grid is very accurate and repeatable to make. Unfortunately, it makes for a poor back-lit tablet because the grid lines are wide enough to cause shadows which are hard to diffuse, and the FR4 material is a poor conductor of light.
Another method of making grid arrays is to print a conductive ink on non-conducting material. However, to achieve sufficient reliability and conductivity, the printed grid lines are very wide and produce shadows that are very hard to diffuse, if used in a back-lit design. For these reasons, some manufacturers make their back-lit tablets using, what is known in the industry as, "strung wire" grids. These grid arrays are made of fine wires which are glued/potted in place on a transparent or translucent sheet.
The fine wire grid creates smaller shadows which are more easily diffused, but this grid making technique has manufacturing problems. Strung wire grids tend to be labor intensive to build and prone to flaws. One kinked or slightly misplaced grid wire can cause a completed digitizer tablet to be scrapped, or sold as a low accuracy model. This is especially a problem because the typical customer requiring a back-lit digitizer, usually requires the highest accuracy digitizer available.
Some digitizer designs have such a dense grid of opaque conductors, it is impractical to back-light.
Digitizing tablets are made of electronic and mechanical components. While the electronic components may be identical in a standard opaque digitizer and a back-lit digitizer, there are great differences in the mechanical components between the two types of digitizers. It is easy to see, in the U.S. Pat. No. 4,943,689 that the mechanical components related to the back-lighting are quite substantial as compared to the mechanical components related to the grid array section that would be used only in an opaque digitizer. The added mechanical complexity and cost of back-lighting was not so serious a marketing concern when the cost of the electronics was a major part of the cost of the digitizer. However, in recent years, the cost of the electronic components in a digitizer has fallen drastically. This has been followed by a similar reduction in the list prices of opaque digitizers.
However, as the prices of opaque digitizers have fallen, the prices of back-lit digitizers have changed little. The added mechanical complexity and cost of back-lighting has become a major cost in a back-lit digitizer. This can be seen in pricing as back-lit digitizers are now typically double the price of an equivalent opaque digitizer.
Back-lit tablets typically use transparent or translucent materials to glue the layers together and/or hold the grid wires in place. With age, these materials have been known to yellow or darken, so that the illumination of the working surface is dimmed.
Surface-lit digitizers, with optical fibers, as described in U.S. Pat. No. 5,001,306 (Purcell) and U.S. Pat. No. 5,153,386 (Siefer and Purcell), were developed in an attempt to overcome the problems of back-lit digitizers. However, these surface-lit digitizers have failed to be a great market success because they are expensive to make and impractical to build in sizes greater than 17.times.24 inches. This is quite a disadvantage since the most popular back-lit digitizers are 24.times.36 inches and larger. These fiber optic surface-lit tablets are complex structures to build. They consist of several layers of materials which must be carefully laminated to avoid optical defects. At least one layer of such materials is a layer of a plurality of discrete optical fibers. Although these surface-lit designs do not have to illuminate through a grid of opaque conductors, the light is emitted from discrete fibers. These patents describe the use of a diffuser layer placed over the discrete fibers to give even lighting. The diffuser reduces the light efficiency of the system, which requires the use of added candlepower for the desired illumination level. The diffuser also adds to the thickness of the light panel, which places the digitizing transducer further from the tablet grid array. This added distance is undesirable because it can reduce the accuracy of the digitizer.
Another system for surface-lit lighting employs electroluminescent lighting, as described in U.S. Pat. No. 5,153,386, does not appear to have been commercially marketed perhaps due to the light intensity being inadequate to generate sufficient illumination to back-light a paper map, and the non-white color may be objectionable. Digitizer tablets require a flat, incompressible, and stable work surface for best digitizing accuracy. Back-lit and surface-lit digitizers as are generally known to the art, are made from layers of materials. The layers are often constructed with deliberate air gaps to prevent undesirable optical effects such as dark spots or rainbows in the illuminated work surface. This layering with air gaps tend to make the work surface flexible and compressible, which may reduce digitizer accuracy.
The foregoing disadvantages are overcome by the unique illumination apparatus for a digitizer tablet of the present invention as will be made apparent from the following description thereof. Other advantages of the present invention over the prior art also will be rendered evident.